Numerous embodiments of horizontal furnaces of this type are known which are generally distinguished from one another by their devices for coupling and driving of the rollers and by their roller bearings. Known examples of horizontal furnaces of this type are described, for example, in European Patent Publication Nos. 51,539 and 70,244; in German Patent Publication Nos. 2,319,049 and 2,605,303; and in U.S. Pat. Nos. 4,230,475; 4,399,598; and 4,725,300.
In the hot zone of such horizontal furnaces, the carrying rollers or cylinders forming the conveying track usually consist of cylindrical ceramic rods or tubes the end sections of which are brought through the lateral walls of the furnace to the outside of the furnace and which rotate in bearings placed outside the furnace. Rotation of the carrying rollers in bearings placed in a furnace is not possible with known means because temperatures of more than 600.degree. C. prevail there. The carrying rollers are suitably coupled on the outside of the furnace to a drive device and are driven synchronously.
Various embodiments are known for rotating carrying rollers on the outside of the furnace.
According to the teachings of U.S. Pat. No. 4,399,598, metal caps are pressed on the two ends of the ceramic tubes, and each metal cap carries a pin end which rotates in an ordinary ball bearing.
According to the teachings of European Patent Publication No. 51,539, the end sections of the carrying rollers are each mounted on the outside of the furnace in free rotation on two steel rollers placed a short horizontal distance from one another.
Further, according to German Patent Publication Nos. 2,319,049 and 2,605,303, the end zones of the carrying rollers located outside the furnace are placed on belts driven synchronously, and the belts, for their part, slide on planar support surfaces. In the direction of movement of the belts, behind the carrying rollers, are placed stops or stop rollers against which the carrying rollers roll and which assure that the carrying rollers retain their position during the rotation movement. Driving of the carrying rollers is performed by static friction, created under the effect of the weight of the rollers, between the belts in movement and the carrying rollers placed on the belts.
To preserve the optical properties of the glass sheets, the support lines of the rollers should exhibit the smallest possible spacing in the zone of the furnace where the glass sheets reach or exceed their softening temperature. This means that the carrying rollers should have the smallest possible diameter. In practice, carrying rollers are used which have a diameter of 4 to 5 cm, spaced closely together.
As the width of the furnace increases, which is necessary for heating or treating of glass sheets with large dimensions, it becomes more difficult to maintain the high optical quality desired for the glazings. This results from the fact that, as their length increases with an unchanged diameter, the carrying rollers undergo an increasing elastic bending due to their own weight. (In the worst cases, the carrying rollers undergo permanent deformations.) Consequently, the upper generatrices of the carrying rollers are no longer optimally in the same plane, and this more or less uneven support of the glass sheets results in the degradation observed in the surface evenness of the glazings and thus in an alteration of the optical properties.
A glass annealing frame is known from German Patent Publication No. 615,421 in which the roller bearings are placed inside the furnace, but in this type of furnace a notably lower temperature prevails than in a bending-tempering furnace. With known means, it is not possible to remedy the drawbacks described above by placing the bearings of the carrying rollers in the heating enclosure.